In a camera-based multitouch display, the cameras are not always placed exactly as designed. This is caused e.g. by finite mechanical tolerances that cause some variation in the physical setup of multitouch display systems. Hence, for optimal performance, it is necessary to calibrate each display separately, for each of its cameras. The parameters to calibrate involve:
camera sensor region of interest (ROI), i.e. the area of the camera sensor that is used for touch tracking;
camera lens distortion parameters (such as barrel distortion);
focus point for performing anti-distortion calculations; and
offset of the image from desired target location.
FIG. 1a shows various errors in images of 16 cameras. In FIG. 1a, each camera is designed to be centrally located so that the cameras form a regular grid. While the camera images of the bottom row should be rather similar, differences in lens placement and lens properties make the images vary as is illustrated by FIG. 1a. FIG. 1b presents a more optimized view of the same camera setup, after per camera region of interest (ROI) adjustment, to achieve more even coverage of the display screen.
The camera ROI placement can be adjusted per-camera in order to account for the physical differences between different cameras. Also camera distortion parameters can be optimized per camera, as illustrated by in FIGS. 2 and 3. FIG. 2 shows an object seen by a camera before distortion parameter optimization and FIG. 3 shows the object after the distortion parameter optimization.
Recognition of pointing objects from an infra-red based multitouch display depends on the quality of the operation of the cameras. Camera errors increase difficulty of accurately recognizing boundaries objects that point the touch screen and of determining when an object actually touches the display.